1. Field of the Invention
The present invention relates to an optical information recording medium of high recording density and a recording method thereof.
2. Prior Art
Recently, an optical disk has been developed for use as an information recording medium. FIG. 1 shows the structure of a prior art optical disk, wherein reference numeral 10 designates a substrate of the optical disk while reference numeral 20 designates a bit record cell formed as an embossed pit or a heap-bump on the substrate 10. The bit record cells 20 are formed on a track line. The bit record cells 20 are arranged at interval which are longer than a minimum read distance at which a laser optical head can detect each cell separately.
The optical disk can be used as an optical information recording medium. When information is recorded on an optical disk, an intense laser beam selectively radiates bit record cells 20 so as to heat them. This optical disk is advantageous because recording of a high S/N ratio and low jitter can be realized easily. That is, bit record cells 20 of equal size each in correspondence to a bit of information are formed on the substrate 10 as pits or bumps independently of each other so that each bit record cell is isolated thermally from the others. Therefore, when an intense laser beam pulse radiates a cell 20, the laser be&m does not affect adjacent bit record cells, and signals can be recorded only at desired bit record cells. Thus, fluctuations of record marks which become a factor with respect to jitter and noise when recording with laser pulse heating rarely occur.
However, a laser beam should be located at the centers of the bit record cells 20 to be recorded when information is recorded with laser beams; otherwise, the laser beam may radiate between two adjacent bit record cells, thereby resulting in two bits being recorded incompletely. Therefore, it is necessary for a laser head to recognize the positions of the bit record cells which are to be recorded correctly before the information is recorded. In prior art disks, the positions are recognized from a change in the amount of reflected light when the laser beam scans the bit record cells 20. That is, as shown in FIG. 2, the amount of reflected light attains a minimum value (P in FIG. 2) when the laser beam scans around the center of a bit record cell 20. Then, clock signals are derived from the detected minimums of the signal and the laser beam is emitted intensely at the appropriate timings. Thus, information can be recorded surely on the bit record cells 20 so that the correct recording of information can realized (see for example, U.S. Pat. No. 4,811,331).
However, such a prior art optical disk has problems with respect to the enhancement of the S/N ratio of the optical disk and the recording density thereof. The recording density has an upper limit because each bit record cell cannot be recognized if the distance between adjacent bit record cells becomes smaller than the above-mentioned minimum read distance. That is, the bit record cells must be formed at intervals which are sufficiently wider than the minimum read distance. Otherwise the amount of reflected light does not change at all when the bit record cells are scanned. Thus, the position of each bit record cell 20 cannot be independently identified and information cannot be recorded correctly on the bit record cells 20.
A similar problem arises when the track pitch or the distance between adjacent tracks is narrowed. If the track pitch becomes narrower than the minimum read distance, the detection between two adjacent tracks or the detection of tracking error signals becomes impossible in principle, so that the tracking control necessary for recording and reading information cannot be performed.